In recent years, as growing importance is attached to resource saving and environmental protection, the demand for a reduction in the fuel consumption of automobiles has become increasingly stronger. Also for automobile tires, it is desired to reduce their rolling resistance and thereby contribute to a reduction in fuel consumption. In order to reduce the rolling resistance of tires, it is common practice to use, as the rubber material for tires, a rubber material which can yield a vulcanized rubber showing a low degree of heat build-up.
Conventionally, it has been proposed to reduce heat build-up by using, as the rubber material for tires, a rubber composition comprising a diene rubber into which, in place of carbon black, silica is incorporated as a reinforcing agent. However, as compared with carbon black-filled rubber compositions, such silica-filled rubber compositions have the disadvantage that they fail to achieve sufficient abrasion resistance and tensile strength. One of the causes therefor is believed to be that silica has a lower affinity for diene rubbers than carbon black and hence fails to exhibit a sufficient reinforcing effect.
Conventionally, a method for enhancing the affinity of silica for diene rubbers by using a silane coupling agent has been proposed (Japanese Patent Laid-Open No. 252431/'91. Japanese Patent Laid-Open No. 252433/'91, etc.). However, in order to achieve a satisfactory effect, this method requires the use of a large amount of an expensive silane coupling agent.
As another improvement, the use of a diene rubber into which a substituent group having an affinity for silica has been introduced is being investigated. For example, diene rubbers having a tertiary amino group introduced thereinto (Japanese Patent Laid-Open No. 101344/'89) have been proposed for diene rubbers formed by emulsion polymerization; and diene rubbers having introduced thereinto an alkylsilyl group (Japanese Patent Laid-Open No. 188501/'89), a halogenated silyl group (Japanese Patent Laid-Open No. 230286/'93) or a substituted amino group (Japanese Patent Laid-Open No. 22940/'89) have been proposed for diene rubbers formed by anionic polymerization.
However, most of the diene rubbers having the aforesaid substituent groups introduced thereinto show poor processability because, when they are mixed with silica, they cohere strongly with silica and cannot be dispersed satisfactorily. Moreover, they also have the disadvantage that their properties such as heat build-up, tensile strength and abrasion resistance are not fully improved.
An object of the present invention is to provide a rubber composition containing a diene rubber component and a reinforcing agent, and capable of yielding a vulcanized rubber which shows a low degree of heat build-up, exhibits excellent tensile strength and abrasion resistance, and has good processability.